Add compatibility with Rails 6.

[expeehaa]

This branch fixes #33.
It also drops support for Rails 3 and 4 because I had difficulties running appraisal install with these old Rails versions and figured that they shouldn’t be supported anymore anyway.

[olliebennett]

This branch has been working well for me for months; would love to see it merged in if possible! 👍 Thanks

In the meantime, myGemfile contains;

# Nested layouts
# Using GitHub PR branch until https://github.com/rwz/nestive/pull/34 is merged
gem 'nestive', github: 'expeehaa/nestive', branch: 'add_compatibility_with_rails_6'

[rwz]

Any reason this drops 3.1 support? It seems to be that it should be pretty straightforward to retain it.

Edit: Oh nevermind, actually read your PR comment, lol. 🤦‍♂️